fix: prefer explicit dates in Tavily searches

[Last-emo-boy]

Fixes #8154

Modifications / 改动点

• Prefer explicit start/end dates when a Tavily request also contains time_range.

• Preserve the existing time_range behavior when no explicit date is provided.

• Add regression coverage for both dates, either single date, and empty dates.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

• uv run pytest -q tests/unit/test_web_search_tools.py: 27 passed
• ruff format --check .: passed
• ruff check .: passed

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Checklist / 检查清单

• This change is covered by the linked issue.
• My changes have been well-tested, and verification results are provided above.
• No new dependencies are introduced.
• My changes do not introduce malicious code.

Summary by Sourcery

Normalize Tavily web search date filters to prefer explicit start/end dates over time_range while preserving existing behavior when no valid dates are provided.

Bug Fixes:

• Ensure Tavily searches use provided start/end dates instead of time_range when explicit non-empty dates are given.
• Fall back to time_range when start/end date inputs are empty or whitespace-only, avoiding invalid date payloads.

Tests:

• Add parametrized async tests covering Tavily date filter normalization for time_range-only, explicit dates, single-date, and empty/whitespace date cases.

[gemini-code-assist]

Code Review

This pull request updates the Tavily web search tool to prioritize start_date and end_date over time_range when constructing the search payload, and adds comprehensive unit tests for this normalization logic. The feedback recommends stripping whitespace from start_date and end_date to prevent whitespace-only strings from being incorrectly treated as truthy values, which could cause API errors.

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[gemini-code-assist]

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[sourcery-ai]

Hey - I've left some high level feedback:

• The allowed time_range values ("day", "week", "month", "year") are now duplicated inline in this logic; consider centralizing them as a shared constant or enum so future changes don’t require hunting through call sites.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The allowed `time_range` values (`"day"`, `"week"`, `"month"`, `"year"`) are now duplicated inline in this logic; consider centralizing them as a shared constant or enum so future changes don’t require hunting through call sites.

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[Last-emo-boy]

Reviewed: the allowed time_range list is not duplicated by this change; the existing check was moved unchanged into the mutually exclusive branch. Keeping this single-use validation inline follows the repository Inline-First rule.